A carbostyril derivative having the general formula (1): ##STR2## wherein R.sup.1 is hydrogen or a lower alkyl group optionally having hydroxy group as a substituent; R.sup.2 is a phenyl(lower alkyl) group optionally having 1 to 3 substituents selected from the group consisting of a lower alkoxy group and a halogen atom on the phenyl ring, pyridyl(lower alkyl) group, or a group of the formula --A--NR.sup.3 R.sup.4 (wherein A is a lower alkylene group, R.sup.3 and R.sup.4 are the same or different and represent a lower alkyl group or phenyl group), or a salt thereof, is a known compound which is described in, for example, EP Publication No. 355583 to be useful as cardiotonics. These compounds are also known to be useful as antihistamines (Japanese Patent First Publication No. 8319/1981). In addition, among the compounds represented by the above general formula, 6-[3-(3,4-dimethoxybenzylamino-2-hydroxypropoxy)-2-(1H)-quinoline compound (the compound of the formula (1) wherein R.sup.1 is hydrogen, R.sup.2 is 3,4-dimethoxybenzyl group; OPC-18790) has an inotropic activity and is useful as an agent for treating heart failure (The Japanese Journal of Pharmacology, The 64th Annual Meeting, March 1991, Kobe, Japan and International Congress of New Drug Development, August 1991, Seoul, Korea).
The present inventors have intensively studied the above-mentioned carbostyril derivative and a salt thereof, and as a result, have found that said compound shows an excellent platelet aggregation inhibitory activity, phosphodiesterase inhibitory activity, cerebral blood flow increasing activity and platelet aggregate dissociation activity, and is useful as an agent for treating thrombosis and as a phosphodiesterase inhibitor. Thus, the present invention has been completed.